Improve error message for inline vals with non-constants

Closes #11854

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/InlineVals.scala

@@ -4,6 +4,7 @@ package transform
 
 import dotty.tools.dotc.core.Contexts._
 import dotty.tools.dotc.core.Decorators._
+import dotty.tools.dotc.core.Symbols._
 import dotty.tools.dotc.core.Flags._
 import dotty.tools.dotc.core.Types._
 import dotty.tools.dotc.transform.MegaPhase.MiniPhase
@@ -37,7 +38,10 @@ class InlineVals extends MiniPhase:
           if !isPureExpr(rhs) then
             val details = if enclosingInlineds.isEmpty then "" else em"but was: $rhs"
             report.error(s"inline value must be pure$details", rhs.srcPos)
-        case _ =>
-          val pos = if tpt.span.isZeroExtent then rhs.srcPos else tpt.srcPos
-          report.error(em"inline value must have a literal constant type", pos)
+        case tp =>
+          if tp.derivesFrom(defn.StringClass) || defn.ScalaValueClasses().exists(tp.derivesFrom(_)) then
+            val pos = if tpt.span.isZeroExtent then rhs.srcPos else tpt.srcPos
+            report.error(em"inline value must have a literal constant type", pos)
+          else
+            report.error(em"inline value must contain a literal constant value.\n\nTo inline more complex types consider using `inline def`", rhs)
   }

tests/neg/i11854.check

@@ -0,0 +1,24 @@
+-- Error: tests/neg/i11854.scala:4:14 ----------------------------------------------------------------------------------
+4 |inline val j: Int = 2 // error
+  |              ^^^
+  |              inline value must have a literal constant type
+-- Error: tests/neg/i11854.scala:5:14 ----------------------------------------------------------------------------------
+5 |inline val b: Boolean = true // error
+  |              ^^^^^^^
+  |              inline value must have a literal constant type
+-- Error: tests/neg/i11854.scala:6:14 ----------------------------------------------------------------------------------
+6 |inline val s: String = "" // error
+  |              ^^^^^^
+  |              inline value must have a literal constant type
+-- Error: tests/neg/i11854.scala:7:18 ----------------------------------------------------------------------------------
+7 |inline val bagA = new Bag(Seq('a', 'b', 'c')) // error
+  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |                  inline value must contain a literal constant value.
+  |
+  |                  To inline more complex types consider using `inline def`
+-- Error: tests/neg/i11854.scala:8:23 ----------------------------------------------------------------------------------
+8 |inline val bagB: Bag = new Bag(Seq('a', 'b', 'c')) // error
+  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |                       inline value must contain a literal constant value.
+  |
+  |                       To inline more complex types consider using `inline def`

tests/neg/i11854.scala

@@ -0,0 +1,8 @@
+class Bag(seq: Seq[Char])
+
+inline val i = 2
+inline val j: Int = 2 // error
+inline val b: Boolean = true // error
+inline val s: String = "" // error
+inline val bagA = new Bag(Seq('a', 'b', 'c')) // error
+inline val bagB: Bag = new Bag(Seq('a', 'b', 'c')) // error